Circuits have been developed in the past to provide automatic switching of a primary power supply voltage to a battery powered backup power supply when the primary power source has failed. Generally the batteries used with these circuits have had a limited useful life which was much less than the useful life of the circuit that they were attached to. Therefore these batteries, which required periodic checking and replacement, were designed to be removed during the useful life of the circuit.
Recently, however, long-life lithium energy cells have been developed which have a shelf life of many years and which hold the promise of eqaling or exceeding the useful life of their associated circuit requiring backup. For example, it is now possible to buy static RAMs which incorporate their own lithium cells into a single module, which lithium cells are expected to provide backup power for ten years or more.
The lithium energy cells are relatively small compared to other, older battery types; and it is possible to combines a lithium energy cell and a power controller into a single modular package. This power controller/lithium energy cell module can then be mounted onto a printed circuit board containing electronic circuitry to provide battery backup at the printed circuit board level.
At the printed circuit board level, however, at least one problem develops with the use of encapsulated power controllers with lithium energy cells which does not occur with static RAMs which incorporate their own lithium cell into a single module. The standby current required of a lithium cell on a printed circuit board may be several orders of magnitude higher than that required of a single static RAM, thus appreciably lessening the amount of time that the lithium cell can provide backup current to the electronic circuitry on the printed circuit board. As a result, the time that a printed circuit board sits in inventory before being installed into a system may cause a significant discharge of the battery. Until the time that the board is actually placed into operation, the printed circuit board usually does not require that the backup battery provide power to the board, and an appreciable portion of the energy originally stored in the battery may be wasted during this storage.
Therefore it can be appreciated that a power controller which switches a backup battery voltage onto the power supply terminal of an electrical circuit when the primary power source fails and which can also isolate the battery from the electrical circuit after receipt of an isolation command is highly desirable.